13.4 Fungal toxins: food contamination and deterioration

We all know that a few species of mushrooms are poisonous and many fungi
produce secondary metabolites, some of which are known as fungal toxins (see the
section on Secondary metabolism in Chapter 10;
CLICK HERE to view the page). The adaptive significance of toxins in fungi
has been given rather little attention, though it is evident that toxin
production is scattered across the entire Kingdom and must have evolved
independently several times in evolutionary history. An obvious possible
function for toxins is to act as a deterrent to the many animals that might
otherwise eat the mushrooms or other fungal structures (see the section on
Fungi in food webs in Chapter 11;
CLICK HERE to view the page)
(Wong, 2013; Singh et al., 2014).

Wild opossums (Didelphis virginiana) became ill after eating the
toxic mushroom Amanita muscaria (Fly Agaric), and
subsequently developed an aversion to the fungus (Camazine, 1983). It has been
argued that such a function leads to the expectation that poisonous mushrooms
should signal their hazard in some way, and the ‘red cap with white spots’ of
A. muscariacould be cited as an example of
warning colouration. However, this does not seem to be the case. Analysis of
ecological and morphological traits associated with edible and poisonous
mushrooms showed that the poisonous ones were not more colourful
than edible mushrooms, but were more likely to have distinctive odours
(and perhaps flavours). Raising the interesting possibility that poisonous
mushrooms have evolved warning odours/flavours as antifeedants
to enhance avoidance learning by fungivores, in contrast to the warning
colouration used by poisonous animals to signal ‘avoid me’ to potential
predators (Sherratt, Wilkinson & Bain, 2005). There are dangers in this line of
argument: it is close to being anthropomorphic (viewing animal behaviour in
human terms), and it may be taken to imply that fungal toxins are produced only
by mushroom fungi and/or that fungal toxins are aimed specifically at animals.
The fly agaric is a remarkable mushroom in many respects. With its relative
A. pantherina it is responsible for poisoning in man characterised by
central nervous system dysfunction, with ibotenic acid and muscimol being the
active components. However, A. muscaria contains other substances
responsible for psychotropic effects in man, and it has been used since ancient
times for mystical purposes by witch doctors and shamans (Michelot &
Melendez-Howell, 2003; Wasson, 1959; Dugan, 2011; Yamin-Pasternak,
2011 ).

The most important toxins in terms of contamination of human food are the
aflatoxins. These toxins are produced by the filamentous Ascomycota
Aspergillus flavus and A. parasiticus (and less frequently by
several other species of Aspergillus) as secondary metabolites when the
fungi grow saprotrophically on stored food products at temperatures between 24
and 35° C, and moisture contents higher than 7% (10% with ventilation).
Food products likely to be contaminated with aflatoxins include
cereals, such as maize, sorghum, pearl millet, rice, wheat, oilseeds like
groundnut (peanut), soybean, sunflower, cottonseeds, spices including chillies,
black pepper, coriander, turmeric, and ginger, tree nuts such as almonds,
pistachio, walnuts, coconut and pecans. Because the milk of animals fed on
contaminated crops can also contain high concentrations of aflatoxin, dairy
products from farm animals can also be contaminated.

The contamination occurs when the mould fungus grows on the crop and
secretes the aflatoxin, either through its growth in the field or during
post-harvest storage. According to FAO estimates, 25% of world food crops are
affected by aflatoxins each year (Moretti et al.,
2017).

In well-developed countries, aflatoxin contamination rarely occurs in foods
at levels that cause acute aflatoxicosis in humans, but
Williams et al. (2004) conclude that about 4.5 billion people (that’s
about two-thirds of the human population) living in developing countries are
chronically exposed to largely uncontrolled amounts of the toxin.
Aflatoxin contamination of grain consequently poses a major threat
to human and livestock health, and aflatoxin content of the diet is at least
associated (and may cause) liver cancer. It is no coincidence
that liver disease is a common health problem in areas where aflatoxicosis is
rife
(Kumar et al., 2016; Umesha et al., 2016; Vettorazzi & López
de Cerain, 2016).

So, why is Aspergillus attacking humans with aflatoxins? The truth
is, of course, that there is no way that aflatoxin production could have evolved
in Aspergillus in the time (maybe a few thousand years) that humans
have been developing agriculture to the point where we store large amounts of
cereal grains and other crops. The biologically important animals in this story
are not humans but rodents. About 40% of mammal species are
rodents and they cause billions of dollars in lost crops every year because they
collect seed grains into stores in their burrows and nests. Rodents first appear
in the fossil record towards the end of the Paleocene epoch, 65 to 55 million
years ago, and that’s plenty of time for Aspergillus to start competing
with the rodents for ‘ownership’ of their grain stores by
producing highly toxic feeding deterrents!

Humans and furry little mammals are not the only animals that eat fungi (see
the section on Fungi in food webs in Chapter 11;
CLICK HERE to view the page)
(Singh et al., 2014), and many toxic secondary metabolites are
targeted at invertebrates. For example Seephonkai et al. (2004)
describe a glycoside from the insect pathogenic fungus Verticillium
that is cytotoxic toward animal cells. And in a different type of investigation,
Nakamori & Suzuki (2007) show that the cystidia of fruit bodies of Russula
bella and Strobilurus ohshimae defend the fruit body against
Collembola, producing (unknown) compounds that kill arthropods on contact.

Similar to these compounds are the HMG-CoA reductase inhibitors
that were isolated from Pythium and Penicillium cultures in
the 1970s by researchers who:

' …hoped that certain microorganisms would produce such compounds as a
weapon in the fight against other microbes that required sterols or other
isoprenoids for growth...' (Endo, 1992).

The compounds that were isolated became known as the statins;
mevastatin, lovastatin, simvastatin, and pravastatin, now marketed around the
world as effective and safe cholesterol-lowering drugs that are probably the
most widely used pharmaceuticals at the moment. Note that quotation: 'as a
weapon in the fight against other microbes'; these compounds are obviously toxic
to their target competing species but clearly beneficial to humans. What
constitutes a fungal toxin is a matter of definition.

And then there are the strobilurins. Fungi that produce
strobilurins (and the related oudemansins) are found all over the world in all
climate zones, and with only one exception (Bolinea lutea, an
ascomycete) all belong to the Basidiomycota. Strobilurins and oudemansins
inhibit the mitochondrial respiratory chain of fungi
by binding to the ubiquinone (= coenzyme Q) carrier that carries electrons to
the cytochrome b-c1 complex (described in the section The endomembrane
systems of Chapter 5; CLICK HERE
to view the page;
CLICK HERE to view the Virtual Cell Animation describing the
Mitochondrial Electron Transport Chain).

Fungi that produce strobilurins have a modified amino acid sequence in the
binding envelope of the coenzyme Q protein that greatly reduces its binding
affinity for strobilurin, and make the strobilurin-producer
strobilurin-resistant.

At present there are about eight synthetic strobilurins in the fungicides
worldwide market that are used against a range of fungal diseases in various
agricultural crops. Strobilurins now hold an approximate 20% share of the world
fungicide market (Balba, 2007). This entire market is based on fungal toxins
that are toxic to other fungi but have what is described as ‘outstanding
environmental tolerability’ meaning that they have negligible
effects on all other organisms.

One final facet of the fungal toxins story is their potential as weapons;
either the biological component (the fungus) or the specific chemical component
(the active toxin itself). Paterson (2006) argues that the low molecular
weight toxins rather than the fungi themselves are the biggest threat as
bioweapons. Although none are yet known, or even suspected, to have been
‘weaponised’, it is necessary to be aware of the potential threats so that
treatment and decontamination regimes can be developed in advance. As with most
threats, it’s better to be prepared than paranoid (Zhang et al., 2014).